Three major areas of iron metabolism will be covered in our proposed research project. (1) Characterization of membrane receptor sites for transferrin. Ferritin tagged antibodies to transferrin will be utilized in conjunction with electron microscopic studies to determine whether endocytosis of transferrin is involved in the delivery of iron to and from reticulocytes and isolated intestinal epithelial cells respectively. Identification of receptor sites for transferrin will be done utilizing I125 labeling of surface membrane proteins followed by solubilization and gel electrophoresis. Dynamic studies in cells obtained from animals with acquired and congenital disorders of iron metabolism will be studied to determine the role of receptor sites for transferrin in the regulation of iron transfer. (2) Metabolic abnormalities in iron deficient red cells. The role of iron and its relationship to selenium balance in the maintenance of red cell glutathion peroxidase will be studied. The enzyme will be isolated following in vivo administration of Fe59 to determine if iron is an essential component of the enzyme. Selenium absorption and excretion will be measured in iron deficient rats in view of the previous demonstration of selenium as a component of glutathione peroxidase. An attempt will be made to correlate glutathione peroxidase activity and red cell membrane plasticity with abnormalities in the lipid and protein structure of the red cell membrane. (3) The role of excess iron in the pathogenesis of hemochromatosis. An attempt will be made to demonstrate an increase in collagen synthesis in hepatic and myocardial cells in direct response to in vivo and in vitro iron loading. Collagen synthesis will be measured directly by the incorporation of C14 proline into collagen as well as by changes in protocollagen hydroxylase activity.